Larger pieces would make for bigger voids between them, thus less weight by volume, correct?
No what I said is exactly what I meant. I know it sounds counter-intuitive but that's the way it is. Just for example a load of stove which is the largest size coal I deliver will be just about level in the box. Barley even with it's higher water content will barely fit on the truck. Volume wise it would appear to look 1/4 - 1/2 a ton.
Nut and pea are about the same, it's the rice for the most part that takes up more space. Volume wise it would look like about 1/4 of a ton over a 5 ton load. My guess is that although you have larger spaces between the nut you also have more area that is dense product. The rice has more surface area to produce the spaces.